The electrical properties of the heart change dramatically during the first week of development. Of primary importance is the differentiation of specific ion channels. The objective of the proposed research is to describe the conductance properties of these membrane channels and the course of their development. The voltage clamp technique will be used to determine the kinetics, voltage dependence, and activation characteristics of membrane currents which underlie the action potential. The specific charge carriers will be identified by measurements of reversal potentials and peak current magnitudes under conditions employing elevated or reduced concentrations of the permeant ions. Inhibitors of proteins synthesis will be used to determine whether the incorporation of new proteins is required for the differentiation of membrane channels. Methods are described for the dissociation and reaggregation of chick and rat heart cells in the formation of spheroidal aggregates. This preparation has been shown to retain a high degree of spatial homogeneity during voltage clamp conditions and thus is suitable for such an analysis.